1. Field of the Invention
The invention is a functional digital dispenser for microdroplets that has significantly lower material and assembly cost.
2. Background of the Prior Art
Digitally operated microdroplet dispensing devices are well known in the prior art as they are employed in ink jet printing heads. They are usually fabricated with liquid containing channels capped with orifice plates having an array of orifices in fluid communication with the channels. The channels are frequently precision machined from blocks of piezoelectric material which are specially treated with conductive and/or non-conductive coatings which allow individual channels to be xe2x80x9cfiredxe2x80x9d as a result of mechanical and acoustical effects on the liquid contained in the channels created by voltage pulses applied to contacts on the various piezoelectric sidewalls. Some examples of this are Michaelis et al., U.S. Pat. No. 4,887,100 (1989); Pies, Wallace and Hayes, U.S. Pat. No. 5,227,813 (1993); and Pies, Wallace and Hayes, U.S. Pat. No. 5,235,352 (1993) the disclosures of which are hereby incorporated by reference. Although these ink jet printing heads are mass-produced, they remain relatively expensive because of the high cost of the piezoelectric material and processing.
The prior art has recognized the desirability to producing individual pulsed microdroplet ejection devices that are sometimes referred to as xe2x80x9cjettingxe2x80x9d devices. Examples of these individualized electronically controllable pulse droplet ejection devices are found in Zoltan, U.S. Pat. No. 3,863,212 (1972); Keur, U.S. Pat. No. 3,972,474 (1976); Perduijn, U.S. Pat. No. 4,418,354 (1983); Hieber, U.S. Pat. No. 4,828,886 (1989); and Hayes et al., U.S. Pat. Nos. 5,053,100 (1991), and U.S. Pat. No. 3,946,398 the disclosures of which are incorporated by reference herein.
The heart of the individual devices is a very fine circular capillary tube drawn to tiny orifice and surrounded by a generally tubular shaped driver positioned around and in operative contact with the capillary tube. The driver device generates a pressure wave in liquid contained in the capillary tube, which produces successive microdroplets in response to electrical voltage pulses. The most practical driver device for such unitary dispensers is cylindrical piezoelectric material which itself may contain or act as a liquid chamber in communication with the liquid to be dispensed. In essence, the piezoelectric material is itself a tube and has heretofore been fabricated by processes such as machining, extrusion or some form of molding. However, the nature of the material makes such specialty tubular cylindrical shaped piezoelectric material relatively expensive.
There are constraints on the adhesive which can be employed to bond the piezoelectric tube to the round capillary tube. The viscosity must be low enough allowing the adhesive to flow into the contact area. It is especially difficult (expensive) to find a suitable low viscosity adhesive if the dispenser is to be used at elevated temperature. These factors also affect cost.
It would be highly desirable to find a structure which substantially reduces the material and fabrication costs of individual dispensers which can be used with a limited supply of fluid in applications where the device itself can be discarded after the fluid is dispensed. One example would be dispensing perfume.
The invention accomplishes the goal of low cost disposability in a device for ejecting microdroplets of fluid materials by utilizing the combination of a flat or flattened capillary tube and a preferably rectangular cross sectioned elongated strip of piezoelectric material which comprises a portion of piezoelectric material severed from a sheet of piezoelectric material. Piezoelectric material in sheet form is available at a small fraction of the cost of conventional cylindrical shaped piezoelectric actuators.
A flat-sided elongated capillary tube having an orifice at one end is provided with a liquid to be dispensed from a capillary tube, a portion of the liquid being in fluid communication with the orifice. The liquid can be provided as self-contained within the flat sided capillary tube. The liquid can also be provided from a separate reservoir which is in fluid communication with the non-orifice end of the capillary tube. An elongated strip of piezoelectric material is bonded to a flat side of the capillary tube and provided with a connection for drive electronics whereby a series of voltage pulses can be cyclically applied to the piezoelectric material thereby causing dimensional changes in the piezoelectric material and acoustical effects which operate on the liquid in the capillary tube such that droplets of the liquid are dispensed from the orifice of the capillary tube in response to the cyclically applied voltage pulses.
In one form of the dispensing device, the strip of piezoelectric material has nearly the same width as the flat side of the capillary tube and lies generally parallel to it and against it. In another form of the dispensing device, the strip of piezoelectric material lies generally perpendicular to a flat side of the capillary tube with one of the narrow edges bonded, preferably near the centerline, to the flat side of the flat-sided capillary tube. When looked at from the front, or in cross section, this latter construction appears in the form of an inverted xe2x80x9cTxe2x80x9d. The flat-sided capillary tube forms the cross bar of the xe2x80x9cTxe2x80x9d and the piezoelectric strip comprises the stem of the xe2x80x9cTxe2x80x9d. A further improvement is provided by the perpendicularly mounted piezoelectric strip. The capillary tube is mounted on a support surface and the upper edge of the xe2x80x9cTxe2x80x9d-shaped structure is stopped against another support surface. The efficiency of movement of the piezoelectric strip with respect to the capillary tube is improved in this configuration when cyclical voltage pulses are applied to the piezoelectric strip of an intensity and duration sufficient to dispense droplets of dispensing liquid from the orifice. Performance of the combination of the flat-sided capillary tube with an orifice in one end and the elongated strip of piezoelectric material bonded to the flat side of the capillary tube has been found to be enhanced if the capillary tube is preferably fixedly mounted within an open sided (xe2x80x9cUxe2x80x9d-shaped) fixture. The fixture does not necessarily have to be open sided, since all that is required is that the piezoelectric material remain unconstrained within the structure. In this form of the combination, it has been found that operating stability is improved and the droplets can be ejected at lower voltages applied to the piezoelectric material.
A still further embodiment is a combination of a fixture having spaced-apart opposing support surfaces, a flat sided capillary tube having an orifice at one end partly disposed on one support surface of the fixture, and a strip of piezoelectric material coupled between the other of the support surfaces of the fixture and a flat side of the capillary tube wherein the strip of piezoelectric material includes a stand-off strip portion having one edge connected to the piezoelectric strip and an opposite edge coupled to the flat side of the capillary tube to permit operation of the capillary tube at elevated temperature without depoling the piezoelectric material which is driving it. For example, the capillary tube and part of the fixture could be mounted in the heating chamber with the inactive stand-off strip portion extending from the heating chamber in direct connection with a corresponding strip portion of active piezoelectric material which is backed by another portion of the fixture or a different fixture and whereby movement of the piezoelectric strip is transferred by the stand-off strip portion directly to a flat side of a capillary tube.
The combination of a flat-sided capillary tube and a strip of piezoelectric material in operative contact with the flat side of the capillary tube, wherein the capillary tube has an orifice and a dispensing liquid to be dispensed, is accomplished by applying cyclical voltage pulses to electrodes on the piezoelectric strip, the pulses being of an intensity and duration sufficient to dispense droplets of dispensing liquid from the orifice. The piezoelectric strip may be edge mounted along one edge of the piezoelectric material generally perpendicular to said flat side of the capillary tube material whereby the strip of piezoelectric material has a free edge opposite the mounted edge, and the dispensing device is operated by applying cyclical voltage pulses to the piezoelectric strip while the free edge of the piezoelectric strip is restrained. The dispenser may be operated by providing a fixture having spaced-apart support surfaces, mounting the flat-sided capillary tube in one of the support surfaces and the step of providing a strip of piezoelectric material in operative contact with a flat side of the tube may be performed by providing a stand-off strip in contact with the flat side of the capillary tube between the piezoelectric strip and the other support surface of the fixture, and supporting the piezoelectric strip by said other supporting surface of the fixture whereby action produced by cyclical voltage pulses applied to the piezoelectric strip is transferred to the capillary tube by the stand-off strip.
The invention also includes a method of constructing a low cost dispensing device of the type having a capillary tube having a dispensing orifice wherein the capillary tube is in intimate contact over an extended portion of its length with a piezoelectric actuator connected to drive electronics in a power supply capable of operating the piezoelectric actuator by means of cyclical voltage pulses with sufficient intensity and duration, wherein the improvement comprises providing the capillary tube as a flat sided tube, providing the piezoelectric actuator as a flat strip of piezoelectric material, bonding the flat sided piezoelectric material to the flat side of the capillary tube and providing electrical contacts to conductive coating on the piezoelectric material whereby droplets of dispensing liquid may be dispersed from the orifice in response to application of said voltage pulses to said piezoelectric material.
The method of construction includes cutting the flat strip of piezoelectric material from a larger sheet of the piezoelectric material which preferably has been previously provided with an electrically conductive coating layer which will serve as a basis for attachment of electrode wires to operate the dispensing device after the flat strip is cut from the sheet. The larger strip of piezoelectric material is cut to produce an intermediate long flat strip of piezoelectric material having the electrically conductive coating and whereby a portion of the electrically conductive coating is removed by suitable process. The piezoelectric actuator is selected from a plurality of said flat strips obtained by cross cutting them from the intermediate long flat strip. The method of operating further includes operating the piezoelectric strip, flat sided tube combination, by providing a xe2x80x9cUxe2x80x9d-shaped support for the capillary tube and operating the dispenser to dispense liquid droplets by means of voltage pulses which are applied to the piezoelectric strip while the capillary tube is supported by the xe2x80x9cUxe2x80x9d-shaped support.